1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus such as a copier, facsimile machine, and printer.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus. In the transfer device, alternatively, the toner image formed on the photoconductor may be primarily transferred onto an intermediate transfer body and then secondarily transferred onto a sheet from the intermediate transfer body.
One longstanding problem of such image forming apparatuses is the unwanted formation of irregular images that have uneven toner density or white spots caused by an uneven image transfer rate due to installation environment or state of use of the apparatus.
To prevent the formation of such irregular images, a method for determining appropriate image forming conditions (that is, determining an appropriate transfer rate) involving detecting the presence of residual toner on the photoconductor using a reflective optical sensor is widely known. In this method, a test pattern is formed on the photoconductor, and residual toner on the photoconductor after the test pattern is transferred onto a sheet or an intermediate transfer body is then detected by the reflective optical sensor to determine the appropriate image forming conditions.
In another approach, a test pattern is formed on an intermediate transfer belt, and then residual toner on the intermediate transfer belt after the test pattern is transferred onto a sheet is detected by a reflective optical sensor to determine the appropriate image forming conditions based on the results detected by the reflective optical sensor.
A problem with the use of reflective optical sensors, however, is that these sensors cannot detect small amounts of residual toner. The reflective optical sensor directs light onto a surface to be detected to detect an amount of residual toner based on an amount of light reflected from the surface to be detected. Specifically, when residual toner is present in a detection range where the light is directed (hereinafter referred to as a target detection range), the reflected light is diffused, an amount of reflective light entering a light receiving element of the reflective optical sensor is reduced, and the output from the reflective optical sensor is reduced compared to a case in which the residual toner is not present in the target detection range. However, when only a slight amount of residual toner is present in the target detection range, an amount of reflective light entering the light receiving element of the reflective optical sensor is not much different from that when the residual toner is not present in the target detection range. As a result, the output from the reflective optical sensor when only a slight amount of residual toner is present in the target detection range is almost the same as that when the residual toner is not present in the target detection range. Consequently, the reflective optical sensor may inadvertently detect that residual toner is not present even when a slight amount of residual toner is in fact present.
In a case in which a test pattern includes a solid patch having a length of several millimeters in a width direction thereof (that is, in a main scanning direction), a certain amount of toner of the solid patch remains as residual toner in the target detection range. Accordingly, the output from the reflective optical sensor is reduced to a certain degree compared to the case in which the residual toner is not present at all in the target detection range, thereby providing more accurate detection of the residual toner.
However, in a case in which the test pattern is formed as a line image having a length of several dots in a width direction thereof, only a slight amount of residual toner is present in the target detection range. Consequently, the output from the reflective optical sensor when only a slight amount of residual toner is present in the target detection range is almost the same as that when the residual toner is not present at all in the target detection range, preventing accurate detection of the residual toner as described above.
Line images are easily affected by uneven image transfer, such that even a slight increase in an amount of residual toner caused by variation in transfer rate can cause irregular images including white spots. Increasing demand for higher-quality images requires image forming apparatuses in which image forming conditions are controllable to prevent the formation of white spots in line images.
However, as described above related-art image forming apparatuses cannot accurately detect residual toner of the line images, thus preventing accurate detection of transfer rates of the line images. Consequently, image forming conditions are not controllable in the related-art image forming apparatuses, causing white spots in the line images.